It is well known that the reactivation of hepatitis B virus (HBV) may occur as an acute hepatitis after chemotherapy or immunosuppressive therapy. Although most of these cases have been reported in HBsAg-positive patients, there have been a few reports of HBV reactivation in HBsAg-negative patients. There have been concerns for the need to screen the reactivation as well as anti-viral prophylaxis in HBsAg-negative patients with possible HBV occult infection who are planning to undergo chemotherapy or immunosuppressive therapy. Rituximab, an anti-CD20 monoclonal antibody, is effective in the treatment of non-Hodgkins lymphoma. However, rituximab can affect the immunity against HBV, consequently increasing viral replication. In fact, there have been reports of HBV reactivation after treatment with rituximab. Here, we report a case of HBV reactivation following rituximab plus systemic chemotherapy in diffuse large B cell lymphoma patient who was HBsAg negative, anti-HBs positive, and anti-HBc positive, ultimately leading to treatment-unresponsive fulminant hepatic failure.